There are many different MEMS devices used today, for example pressure sensors, pump actuators and electrical or optical circuit elements, such as RF inductors, optical switches and resonators, to name but a few.
Many MEMS devices use capacitive circuit elements to perform an intended function. The capacitive circuit elements of such MEMS device are connected by conductors to electronic circuits, which may be placed on the same substrate as the MEMS device, or on a separate integrated circuit. In either case, capacitance associated with the conductors will be present. This capacitance, which is often called parasitic capacitance, will be electrically in parallel with the capacitive circuit elements of the MEMS device.
In addition, capacitive MEMS devices sometimes need to have signals routed via diffused conductors, instead of via metal conductors. An example might be when wafer-scale packaging is used, whereby a top wafer is bonded to the sensor wafer by one of several techniques, such as anodic bonding or direct silicon wafer bonding. Conductors are then required to cross the bonding area below the wafer surface, since a flat wafer surface without metal conductors is required for successful bonding. Another example would be conductors on thin membranes, where using a metal could cause hysteresis. However, the capacitance of the back-biased junction, which is generally employed to isolate the conductor from the substrate, appears as parasitic capacitance in parallel with the sensing capacitance, impairing performance.
Accordingly, MEMS devices, which use resistive or inductive circuit elements to perform their intended function, may also have their performance impaired by the parasitic capacitance. This is because the parasitic capacitance causes reduced sensitivity, increased noise gain and/or reduced sensor bandwidth. It is therefore desirable to reduce the parasitic capacitance present in a MEMS device.
One of the factors that influence the amount of parasitic capacitance is the total area of the diffused conductors. Previously, this problem has been handled by keeping the area of the diffused conductors to a minimum, for example limiting the length of the conductors by placing the associated electronics close to the MEMS device, or using narrow conductors. However, the use of narrow diffused conductors leads to higher resistance, which also has negative effects.
Furthermore, the mechanical dimensions of the MEMS device are often determined by factors other than the electrical parameters, and this restricts the possibility to reduce conductor area. One example of particular relevance is the use of anodic bonding in MEMS devices. This technique requires a bonding area of a certain width, which must be crossed by conductors which carry electrical signals from the hermetically sealed MEMS device to outside electrical contacts.
Although circuit topologies, such as those provided on an electronic amplifier, can differ in their sensitivity to parasitic capacitance, and although several circuits have been suggested as being improvements in this respect, in many cases parasitic capacitance remains detrimental to system performance.